Research On Process Optimization And Properties Of Al_0.3CoCrFeNiCu High-entropy Alloy Additively Manufactured By Selective Laser Melting

The excellent properties of high-entropy alloys play an increasingly critical role in aerospace,biomedical,and transportation industries.The traditional high-entropy alloys manufacture mainly uses vacuum arc melting,induction melting,mechanical alloying and so on.The alloys prepared by these processes are always limited in size and shape,cannot avoid defects like pores,segregation and inclusions,which cannot meet the needs of many complex components in the industry.Selective laser melting,as one of the metal additive manufacturing technologies with high forming accuracy and unlimited product shape,has been successfully used to manufacture complex or personalized metal components.Furthermore,Al_0.3CoCrFeNiCu high-entropy alloy,is the earliest developed composition with excellent strength-plastic comprehensive mechanical properties,fine corrosion resistance and machinability.In this thesis,the pre-alloyed powder of Al_0.3CoCrFeNiCu HEA is used as the material to research the influence of processing parameters on the microstructure and properties of the SLMed HEA systematically.The intrinsic relationship between properties-process-structure of alloy is studied,and the best SLM post-heat treatment process is found to provide.These provied a theoretical basis for SLM forming of complex high-entropy alloy.Firstly,the effects of laser power,scanning speed,hatch distance and scanning strategy on the relative density,microstructure and defect distribution of Al_0.3CoCrFeNiCu high-entropy alloys were investigated.The results show that the scanning speed significantly affects the density of the material.The relative density of the alloy is below 95%at the scanning speed of 300mm/s and 700mm/s,and reach more than 97%at the speed of 500mm/s.At the condition of fixed scanning speed,the relative density of alloys changes little at different laser power parameters.The comparison results of different hatch distance show that the highest density of the alloy is at the 0.06 mm condition,and at higher hatch distance,the reduction of the zone of remelting path leads to more crack defects and lower densities.The comparison results of different scanning strategies show that compared with strip scanning,the former can reduce the internal residual stress and reduce the number of internal defects.The optimal processing parameters are obtained as 150 W of laser power,500 mm/s of scanning speed,0.06mm of hatch distance and partition scanning strategy.Based on these process parameters,the relative density of the alloy can reach more than 99%.The HEA is a single FCC solid solution,and the fine columnar crystals grow epitaxially along the building direction.The typical molten pool with"fish scale"morphology can be seen.The micro-defects mainly include irregular pores caused by unmelted powders and microcracks caused by thermal shrinkage.The yield strength of each sample is more than 360 MPa with good plasticity,and the surface hardness is above 250 HV.Secondly,the microstructure,mechanical properties and corrosion resistance of the Al_0.3CoCrFeNiCu high-entropy alloy prepared by optimal parameters and the traditional casting high-entropy alloy with the same composition were compared.The microstructure of the SLM sample is small columnar grains and a cellular structure with a width of about 1μm arranged in parallel inside the grain,and the latter has a large number of geometrical necessary dislocations（GNDs）caused by thermal shrinkage.A{100}oriented texture is formed in the parallel build direction.The SLM rapid cooling mechanism can eliminate the Cu segregation,and thus greatly improves the corrosion resistance of the alloy.Based on the results of microstructure characterization,the mechanical properties of SLM samples are all better than those of casting alloys.The grain refinement strengthening,subgrain strengthening and dislocation strengthening are considered the strengthening mechanism.Finally,the microstructure and properties of Al_0.3CoCrFeNiCu high-entropy alloys manufactured by SLM with annealing treatment were studied.The alloy with annealing is composed of FCC matrix and coherent L1₂ nanoscale precipitation.The mechanical properties of the annealed sample（600℃/10h）are the most strengthened,and the yield strength and hardness of the original sample are increased from 418MPa and 256HV to 660MPa and 357HV,respectively.The strengthening mechanism is the precipitation strengthening of the L1₂precipitation dominated by the dislocation shearing mechanism.This high-temperature stable precipitation does not change due to the prolongation of annealing time,but the position of the precipitation is different at a higher annealing temperature,so the strengthening effect of annealing at high temperature is weakened.In addition,local metastable pitting and new passivation occurred in the polarization test of the samples made by SLM plus annealing.Therefore,after the polarization test,the surface was smooth without large pits,and the self-corrosion current density is reduced from 7.2×10^-7 A·cm^-2（before annealing）to 6.7×10^-8A·cm^-2（after annealing）,and the corrosion resistance is greatly improved.